[I Buried My Guns]

"...get an upper kit already headspaced for 5.56 NATO as opposed to .223...."

I own 2 AR-15s, one of which I built myself, but I still do not understand the difference between .223 and 5.56 NATO. I thought they were, for our purposes, exactly the same.

[smokingfrog]

You can shoot .223 in your NATO chambered rifle, but don't try shooting 5.56 in a rifle that is chambered strictly for .223 ammo. The NATO dimensions are a little more generous.

( ignore 'N' - I'm pretty sure they're the same )

[Gilbo_3]

they *should* be...but there are rare occassions of the hotter .223 being too much for the 5.56 innards...

what ive been told anyways...

[piytar]

See here: http://www.thegunzone.com/556v223.html

Short version: If your upper is stamped “5.56,” you can shoot both. If it’s stamped “.223,” you risk catastrophic failure due to higher chamber pressure if you shoot 5.56 with it, esp. if it’s an older rifle.

[One Name]

Went to bed last night so maybe this got answered already- 5.56 Nato has a different “lede” or I would say freebore beyond the tip of the chambered round; a longer one which allows barrel pressures around 70,000 copper units. .223 Remington is smaller, rated for around 45-50 cup, don’t recall exactly. You can overpressure a .223 chamber with a NATO round, but not vice versa. You give up a slight amount of accuracy shooting .223 thru a 5.56 barrel. External cartridge dimensions are the same- I think NATO is thicker in the base internally.

Most commercial semi-autos are chambered 5.56 ( Mini-14, etc.) Some bolt actions are not so you have to be careful what you put thru them, especially overall cartridge length if reloading.

Because there are alot of AR shooters after super-accuracy (varminters, especially)they offer the .223 chambering.